The actual molecular site of action of few phytotoxic terpenoids is known. General physiological effects of representatives of various terpenoid categories and methods used to focus mode of action studies on their potential molecular targets are discussed. In particular, methods such as complementation studies, determining effects on mitosis, and measuring effects on plasma membrane integrity are described. The mode of action of 1,4-cineole and how it was discovered are provided in detail. This monoterpene and a metabolic conversion product of its synthetic herbicide analogue, cinmethylin, are potent inhibitors of asparagine synthetase. The sesquiterpene lactone, artemisinin, has an apparently unique yet still unknown mode of action. The sesquiterpene lactone, dehydrozaluzanin C, appears to be generally cytotoxic, causing rapid plasma membrane disruption, perhaps through interaction with sulfhydryl groups of membrane proteins. Quassinoids appear to have a different but still unknown mode of action in plants than in mammalian cells. Saponins may be phytotoxic

© 2004 by CRC Press LLC

through their effects on membrane lipids or by effects on specific enzymes. In summary, much more work needs to be done on the mode of action of phytotoxic terpenoids.

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